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Common Terms Used to Interprete P&ID Drawings

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Interpreting P&IDs can often be very challenging especially for beginners. In this piece, I shall be elaborating on some commonly misunderstood terms used in P&IDs to enable the beginner better understand how to interpret the P&ID drawings of their respective plants.

Computing Device

This is a device or function that performs one or more calculations or logic operations, or both, and transmits one or more resultant output signals. A computing device is sometimes called a computing relay.

Converter

A device that receives information in one form of an instrument signal and transmits an output signal in another form is called a converter. An instrument which changes a sensor's output to a standard signal is properly designated as a transmitter, not a converter. Typically, a flow element (FE) may connect to a Flow transmitter (FT), not to a converter (FY). A converter is also referred to as a transducer; however, "transducer" is a completely general term, and its use specifically for signal conversion is not recommended. An I to P (current to pneumatic) converter is a converter we often come across in P&ID drawings.

Local

This is the location of an instrument that is neither in nor on a panel or console, nor is it mounted in a control room. Local instruments are commonly in the vicinity of a primary element or a final control element. The word "field" is often used synonymously with local.

Local Panel
This is a panel that is not a central or main panel. Local panels are commonly in the vicinity of plant subsystems or sub-areas. The term "local panel instrument" should not be confused with "local instrument." From my explanation on the word local above, a local instrument implies an instrument in the field.

Monitor

A monitor is a general term for an instrument or instrument system used to measure or sense the status or magnitude of one or more variables for the purpose of deriving useful information. The term monitor is very often unspecific when used in P&ID drawings — sometimes meaning analyzer, indicator, or alarm. Monitor can also be used as a verb

Panel
A panel is a structure that has a group of instruments mounted on it, houses the operator-process interface, and is chosen to have a unique designation. The panel may consist of one or more sections, cubicles, consoles, or desks. Panel is the Synonym for board on P&IDs

Panel-mounted
This is the term applied to an instrument that is mounted on a panel or console and is accessible for an operator's normal use. A function that is normally accessible to an operator in a shared-display system is the equivalent of a discrete panel-mounted device.

Pilot light
A pilot light indicates which number of normal conditions of a system or device exists. It is unlike an alarm light, which indicates an abnormal condition. The pilot light is also known as a monitor light.

Sensor
A sensor is that part of a loop or instrument that first senses the value of a process variable, and assumes a corresponding, predetermined, and intelligible state or output. The sensor may be separate from or integral with another functional element of a loop. The sensor is also known as a detector or primary element.

Set point
The set point is an input variable that sets the desired value of the controlled variable. The set point may be manually set, automatically set, or programmed. Its value is expressed in the same units as the controlled variable.

Shared controller
This is a controller, containing pre-programmed algorithms that are usually accessible, configurable, and assignable. It permits a number of process variables to be controlled by a single device.

Shared displayThis is the operator interface device (usually a video screen) used to display process control information from a number of sources at the command of the operator.

Transducer
Transducer is a general term for a device that receives information in the form of one or more physical quantities, modifies the information and/or its form, if required, and produces a resultant output signal. Depending on the application, the transducer can be a primary element, transmitter, relay, converter or other device. Because the term "transducer" is not specific, its use for specific applications is not recommended

Transmitter
This is a device that senses a process variable through the medium of a sensor and has an output whose steady-state value varies only as a predetermined function of the process variable. The sensor may or may not be integral with the transmitter. A transmitter is often required where the instrument signal needs to be sent to a central control room or transmitted through some distance.